Cyanuric acid as a scale reducing agent in coating of zinc surfaces

ABSTRACT

AN ALKALINE COMPOSITION SUITABLE FOR FORMING A COATING ON ZINC AND ZINC ALLOY SURFACES WHEREIN THE IMPROVEMENT COMPRISES EMPLOYING A SCALE REDUCTION AMOUNT OF CYANURIC ACID IN THE COMPOSITION.

United States Patent Oflice 3,756,864 Patented Sept. 4, 1973 CYANURICACID AS A SCALE REDUCING AGENT IN COATING F ZINC SURFACES John BrunoZaccagnini, Warren, Mich., assignor to Oxy Metal Finishing Corporation,Warren, Mich. No Drawing. Filed Sept. 7, 1971, Ser. No. 178,504 Int. Cl.C23f 7/00 US. Cl. 1486.14 R Claims ABSTRACT OF THE DISCLOSURE Analkaline composition suitable for forming a coating on zinc and zincalloy surfaces wherein the improvement comprises employing a scalereducing amount of cyanuric acid in the composition.

This invention relates to the art of forming protective coatings on thesurfaces of zinc and zinc alloys from aqueous alkaline solutions.

Heretofore, it has been well known and widely commercially practiced toprepare the surface of zinc and zinc alloys for the reception of paintor other siccative coatings by using aqueous acidic solutions,particularly phosphate solutions. A number of commercially satisfactoryphosphate coating processes and systems are known and for a certain typeof product manufacture are entirely satisfactory. In other types of zincor zinc alloy products, however, such as those which require deformationof the painted metal surfaces subsequent to painting, certain defectshave been encountered, including loss of paint adhesion, decreasedcorrosion resistance to both humidity and salt spray or its equivalent.

Previously, it has been discovered that corrosion resistant coatings canbe formed on the surfaces of zinc or zinc alloys from certain aqueousalkaline solutions and that these coatings provide bases for paint whichare unexpectedly superior in adhesion to any of the heretofore knowncoatings produced from conventional aqueous acidic solutions includingthe best known phosphates.

The problem that has occurred with these alkaline solutions is that azinc oxide scale has formed in the nozzles of the spray apparatus aswell as in the tanks in which the alkaline solutions are kept duringcoating of the metal surfaces.

In accordance with this invention, it has been found that adherentcorrosion resistance coatings are formed on the surface of zinc and zincalloys by contacting the surfaces for a short time with an aqueous zincoxide alkaline solution and a scale reducing amount of cyanuric acid.

In US. Pat. No. 3,444,007 is described a Zinc oxide coating solution forcoating zinc and zinc alloy surfaces. The contents of that patent arehereby incorporated by reference.

In a preferred form of the present invention, the aqueous alkalinesolution contains at least two metal ions other than, and/ or inaddition to alkali metal ions and a sufiicient quantity of complexingagent to maintain these ions in solution. It has been found that usefulcoatings are produced from the aqueous solutions of this invention whenthose solutions contain a wide range of variation of alkali strength,metallic ion concentration, complexing agent concentration, temperatureand time of processing as well as type of application of the coatingsolution to the surface, each of which component and variations inoperating conditions will be discussed in greater detail hereinafter.

The alkalinity of the solution may be developed by using any of thecommonly available alkaline compounds and alkaline salts, such astriethanola-mine and the alkali metal hydroxides, carbonates,phosphates, borates, silicates, polyphosphates, and pyrophosphates, ormixtures thereof as needed to give the desired pH. It has been foundthat the preferred operating conditions for the use of the solution ofthis invention include the use of a solution which has a pH numericallygreater than about 11, and that better results are obtained fromsolutions having a pH in the range of 12.6 and 13.3. However, someimprovement is obtained from aqueous alkaline solutions having lessalkali strength and lower pH values but the operation conditions forsuch solutions including time to produce the desired coating andtemperature necessary to form the same are less advantageouscommercially than those which are satisfactorily employed when usingsolutions of the greater alkali strength. Using any of the abovedescribed solutions, adherent coatings have been produced which vary incolor from colorless to light yellow, to tanyellow, to brown and finallyto blue-black, depending upon the particular application conditions andsolution "constituency which is being used. The above described coatingsare obtained from solutions which contain extremely small quantities ofmetal ions other than the alkali metal ions and, in general, the colorof the coating increases in intensity toward brown as the time oftreatment is increased and as the concentration of metallic ion thereinis increased, as the temperature of treatment is increased and as thealkali strength is increased.

It has been found that the solution should contain at least one metalion other than an alkali metal ion, which is normally introduced withthe alkaline salts, in order to produce a rate of coating formationand/or quality of coating which enables such coatings to function assuperior bases for paint or other siccative coating. It has also beenfound that such other metal ion is incapable of functioning to increasethe coating rate or coating formation mechanism unless it issufficiently complexed by a complexing agent to dissolve that metal ionin the coating solution. Based on the experimental evidence on hand, itis believed that any metal ion, other than an alkali metal ion which iscomplexed and in solution functions to improve the formation of thedesired coatings since metals falling within each of the groups of theperiodic system including the rare earth metals have been found to besatisfactory for this purpose. Satisfactory results have been obtainedfrom the use of solutions containing the sodium ion and at least oneother ion from the group comprising silver, magnesium, cadmium,aluminum, tin, titanium, antimony, molybdenum, chromium, cerium,tungsten, manganese, cobalt, ferrous and ferric iron and nickel. Asabove indicated, extremely minor quantities of such metal ion provideimprovement. Good results have been obtained from solutions containingas low as about 0.002% iron in addition to the sodium ion introducedinto the solution as sodium hydroxide to form a solution having a pHabove 12. Good results have also been obtained from high concentrationsof the metal ion or ions and although there appears to be no particularadvantage gained from large quantities of the metal ions such excessquantities do not appear to be detrimental and any quantity of metal ionup to the limit of solubility thereof can be used satisfactorily.Somewhat better results have been obtained from the use of solutionswhich contain at least two metal ions other than the alkali metal ion,e.g., iron and cobalt, iron and silver, cobalt and cerium, etc.

The complexing chelating or sequestering agent functions to complex themetal ion other than the alkali metal ion and maintain the samedissolved in the coating solution. For this purpose a wide variety ofcomplexing, chelating, and sequestering agents have been found to besatisfactory. It is suitable to use the inorganic complexacid, etc.; theamino acids, such as glycine; the hydroxycarboxylic acids such ascitric, gluconic, lactic, etc.; the hydroxyaldehydes such as acetylacetone; the polyhydroxyaliphatic compounds such as sorbitol,1,2-ethanediol; the phenolic carboxylic acids such as salicylic acid,phthalic acid; the amine carboxylic acids such as ethylene diaminetetraacetic acid; the polyamino acids such as diethalolaminomethanephosphate; the salts of lower molecular weight lignosulfonic acids suchas derived from wood pulping processes such as sodium lignosulfonate.The proportion of complexing agent which should be present is an amountat least sufficient to completely complex the metal ion other thanalkali metal ion, which is present. It will be apparent that as theconcentration of metallic ion other than alkali metal ion increases theconcentration of the complexing agent will also increase and sinceincreasing quantities of certain of the complexing agent which areacidic in nature tends to reduce the effective alkali strength of thesolution, it is preferred to employ the complexing agent in the form ofneutral salts, particularly the alkali metal salts. Moreover, noadvantage has been observed form the presence of concentrations ofcomplexing agent in excess of that required to maintain the metal iondissolved in the coating solution. Particularly good results have beenobtained from the use of sodium hexahydroxyheptanoate, sodium gluconateand the sodium salt of ethylenediaminetetraacetic acid.

The solutions of this invention may optionally include a compatiblesurface active agent and the presence of such a surface active agent isparticularly advantageous when the surface to be coated is soiled withgrease, oil, or the like. For this purpose, the only requirement for thesurface active agent is that it be compatible with the other ingredientsof the solution, both under normal storage conditions and under theelevated temperature application conditions, that is, the surface activeagent should not cause precipitation or agglomeration of the metal ionswhich are present, cause gelling or itself precipitate or gel in thesolutions. It has been found that there are nonionic, anionic andcationic wetting agents which are compatible in the solutions of thisinvention and when included up to about 5% of the solution, w./v., canbe used.

In general, the process of this invention comprises the step ofcontacting the zinc or zinc alloy surface to be coated by spraying,dipping, brushing, or the like, at a temperature between ordinary roomtemperature and the boiling point of the solution and for a time periodsufficient to produce the desired total coating. The preferred method ofapplication in the formation of coatings adapted for use as a base forpaint is spraying. Preferred conditions for continuous production whichenables the formation of the desired coating in the least time involvesthe use of the solution at a temperature between about 90 F. and 200 F.for a time between about 2 seconds and about 60 seconds. Commerciallysatisfactory coatings have been obtained in about to seconds at about100 F.l60 F. and may be obtained in even shorter periods of time withsolutions containing relatively high concentrations of alkali and othermetal ion and/or at the higher temperatures. Slightly longer contactperiods are required for immersion application, but a satisfactoryquality of coating is obtained from immersion contact times betweenabout 30-90 seconds with solutions having a temperature of about 130Frl80 F.

The complete process of this invention also includes the use of asubsequent dilute aqueous chromic acid rinse on the preliminarily formedcoating of this invention. A suitable solution for this purpose is onewhich contains about 0.01% to 0.5% CrO w./v. or percent by weight perunit volume. When the chromic acid solution is relatively concentrated,it is preferred to remove the excess by squeezing the excess off withrolls. A preferred dilute rinse solution for the purpose of thisinvention is one which is disclosed and described in detail in copendingapplication Ser. No. 230,729, filed Oct. 15, 1962, now US. Fat.

No. 3,279,958, and assigned to the assignee of this application, namely,one which includes a hexavalent chromium and chromium complex ion whichcontains at least about 0.001% w./v. of the trivalent chromium ion andhas a pH within the range of about 3.8 to 6.0, preferably about 4.5.After rinsing the coating may be water rinsed or dried without rinsingas desired and after drying the coating is in condition for thereception of paint or other siccative coating.

The process of this invention has been found to be useful in formingcoatings on the surfaces of pure zinc, electroplated zinc, hot-dippedzinc surfaces including hotdipped zinc containing small quantities ofalloying ingredients such as aluminum, etc., and zinc alloys per se orelectrodeposited zinc alloy surfaces. Where the surface to be coatedincludes iron or steel in addition to zinc or zinc alloy, such as apartially galvanized steel or iron base or a composite article whichincludes assembled steel or iron portions and zinc or zinc alloyportions, it has been found that such solutions of this invention areuniquely useful. For such surfaces the solutions of this invention arefirst applied by the procedures and temperature application conditionsabove specified to first clean the steel or iron portion of the articleand clean and form a coating on the zinc or zinc alloy portion of thearticle, and this step is advantageously combined with a subsequentconventional phosphate coating step. It has been found that the coatingformed on the zinc or zinc alloy portion of such surface is not removedby an aqueous acidic zinc phosphate or aqueous acidic alkali metalphosphate coating solution of any conventional constitution, and thecleaned steel or iron portion of the surface receives an adherentprotective phoshate coating having at least equal corrosion resistanceand utility as a base for paint as conventionally cleaned iron or steelsurfaces. For the purposes of such modified process use it issatisfactory to employ aqueous acidic zinc phosphate solutions oraqueous acidic alkali metal phosphate solutions having any of thepresent-day conventional constitution, which are now well known to thoseskilled in the art.

The composition of the present invention may be prepared by mixing thealkali metal hydroxide with the zinc oxide in water which is heated toabout F. To this mixture is added the sequestering agent plus the ionsother than the alkali metal. This concentrate is then diluted with waterwhich contains the scale reducing agent.

A preferred scale reducing bath composition was prepared by mixing 897grams of sodium hydroxide with 212 grams of zinc oxide in 1255 grams ofwater. This material was heated for a short period of time to about 180F. This mixture then had added thereto 180 grams of Seqlene Fe 1300(sodium hexahydroxyheptanoate), 74 grams of 40% Co(NO '6H O and 52 gramsof Eighty-seven (87) grams of the above concentrate were added to 900milliliters of water which had present therein 0.1% of cyanuric acid.

The following examples will illustrate in greater detail the specificvarient conditions and typical coating solutions of this invention. Butit is to be understood that they are presented for the purposes ofillustration only and do not represent the definite limits of theinvention which have been here and before set forth.

EXAMPLE I Hot dipped galvanized metal surfaces were alkaline cleaned andthen treated with the above scale reducing bath composition. No scaleformation was noted. An equivalent bath composition was used as acontrol except that in place of the cyanuric acid, 5% by weight oftrisodiumphosphate, a commercial product was used. There was no decreasein the formation of the blue zinc oxide scale on the spraying apparatusand the holding tanks.

The above example is an accelerated test to determine the scaledecreasing capability of the additive. In the normal processing of zincsurfaces, no zinc is introduced other than as the metal surface to betreated. In other words, the zinc in the bath is the result of thecoating process.

EXAMPLE II A preferred aqueous alkaline solution was prepared to contain0.40% sodium hexahydroxyheptanoate, 0.40% cobalt added as cobalt nitrate611 0, and 0.041% iron added as ferric nitrate 91-1 0. This bathcontained 0.2% by weight cyanuric acid. The bath had a free alkalinityof 13.3 points (0.1 N sulfuric acid titrated with phenolphthalein asindicator). Electro-galvanized strip was sprayed with the above solutionat 120 F. to produce a contact time of about seconds.

After this procedure was followed for two weeks, the processing tankswere emptied. No indication of scale formation could be detected in thetank using cyanuric acid while scale formation was evident on thesqueegee rolls, pipes and walls of the tanks not employing cyanuricacid.

While cyanuric acid may be used in an amount ranging from 0.05% byweight to about 1% by weight, a preferred amount is from 0.1 to 0.3% byweight.

It is understood that the following claims are intended to cover all ofthe generic and specific features of the invention herein described andall statements of the scope of the invention which, as a matter oflanguage, might be said to fall therebetween.

What is claimed is:

1. An aqueous composition suitable for forming a coating on zinc andzinc alloy surfaces, comprising:

(a) cyanuric acid in a concentration between about 0.1

and 0.3% by weight of the composition, and

(b) an alkaline material in an amount sufficient to effect a pH of thecomposition of at least about 11.

2. The composition of claim 1 wherein said alkaline material is selectedfrom the group consisting of triethanolamine and the alkali metalhydroxides, carbonates, phosphates, borates, silicates, polyphosphates,and pyrophosphates, or mixtures thereof.

3. The composition of claim 1 wherein the amount of alkaline material issufficient to effect a composition pH of between 12.6 and 13.3.

4. An aqueous composition suitable for forming a coating on zinc andzinc alloy surfaces, comprising:

(a) at least one metal ion other than an alkali metal selected from thegroup consisting of silver, magnesium, cadmium, aluminum, tin, titanium,antimony, molybdenum, chromium, cerium, tungsten, manganese, cobalt,ferrous and ferric iron and nickel;

(b) a complexing agent present in said solution in an amount suflicientto hold said other metal ions in said solution;

(0) an alkaline material in an amount suificient to effeet a pH of thecomposition of at least about 11; and

(d) cyanuric acid in an amount suflicient to reduce the rate of scaleformation.

5. The composition of claim 4 wherein said alkaline material is selectedfrom the group consisting of triethanolamine and the alkali metalhydroxides, carbonates, phosphates, borates, silicates, polyphosphates,and pyrophosphates, or mixtures thereof.

6. The composition of claim 4 wherein the amount of alkaline material issuflicient to effect a composition pH of between 12.6 and 13.3.

7. The composition of claim 4 wherein said cyanuric acid concentrationis between about 0.05 and 1% by weight.

8. The composition of claim 7 wherein said cyanuric acid concentrationis between about 0.1 and 0.3% by weight.

9. A method of coating zinc and zinc alloy surfaces comprisingcontacting said surface with an aqueous composition comprising:

(a) cyanuric acid in a concentration between about 0.1 and 0.3% byweight of the composition, and

(b) an alkaline material in an amount sufiicient to effeet a pH of thecomposition of at least about 11.

10. The method of claim 9 wherein said contact is effected by sprayingthe aqueous composition on the surface at a temperature of between and200 F. for a period of from about 2 to 60 seconds.

11. The method of claim 9 wherein said contact is effected by immersingsaid surface in said aqueous composition maintained at a temperature ofabout -180 F. for a period of from about 30-90 seconds.

12. The method of claim 9 wherein said alkaline material is selectedfrom the group consisting of triethanolamine and the alkali metalhydroxides, carbonates, phosphates, borates, silicates, polyphosphates,and pyrophosphates, or mixtures thereof.

13. A method of coating zinc and zinc alloy surfaces comprisingcontacting said surface with an aqueous composition comprising:

(a) at least one metal ion other than an alkali metal selected from thegroup consisting of silver magnesium, cadmium, aluminum, tin, titanium,antimony, molybdenum, chromium, cerium, tungsten, manganese, cobalt,ferrous and ferric iron and nickel;

(b) a complexing agent present in said solution in an amount sufiicientto hold said other metal ions in said solution;

(0) an alkaline material in an amount sufficient to efiect a pH of thecomposition of at least about 11; and

(d) cyanuric acid in an amount suflicient to reduce the rate of scaleformation.

14. The method of claim 13 wherein said alkaline material is selectedfrom the group consisting of triethanolamine and the alkali metalhydroxides, carbonates, phosphates, borates, silicates, polyphosphates,and pyrophosphates, or mixtures thereof.

15. The method of claim 13 wherein said cyanuric acid concentration isbetween about 0.05 and 1% by weight.

References Cited UNITED STATES PATENTS 3,468,803 9/1969 Knapp 252-4062,898,250 8/1959 Pimbley 1486.21 3,137,583 6/1964 Bryan 106-14 2,988,4716/1961 Fuchs 2l2.5 X 3,444,007 5/1969 Maurer et al 1486.15 R

RALPH S. KENDALL, Primary Examiner US. Cl. X.R. 106l4

